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(95), with λ = 1.55 µm). An analogue low-pass filter with a cut-off frequency of 50 MHz,
inserted between the detector and ADC, eliminates aliasing. Spectra are calculated by digital
Fourier transform (DFT) methods; a 512 point DFT gives rise to 256 points in the output
spectrum with a bin width of ∼ 200 kHz, corresponding to a line-of-sight velocity range of
∼ 0.15 m s −1 . Each DFT represents ∼ 5 µs of data; successive DFTs are then calculated,
and the resulting “voltage” spectra are squared in order to generate a power spectrum. These
power spectra are then averaged to find a mean spectrum for the averaging period. The
random fluctuation in the shot noise floor of the spectrum reduces as the square root of
the number of averages: the sensitivity increases by this same factor. For 4000 averages, the
measurement time amounts to ∼ 20 ms (or a data rate of ∼ 50 Hz). This requires that the
processing is capable of 100% duty cycle, which is achieved in ZephIR with a fast Fourier
transform (FFT) block within a field-programmable gate array (FPGA). It has been shown
that a standard PC with no additional duties to perform can achieve a similar performance.
It is possible to accommodate reasonable variations in any of the above parameters (sample
rate, DFT size and number of averages) and maintain the 100% duty cycle.
Figure 45: Stages in typical lidar signal processing: DFT analysis is carried out by a computer
integrated into the lidar system. As an example, 4000 individual spectra might be averaged
to achieve high sensitivity and measurable returns even in very clear air. This entire process
takes only 20 ms, giving ∼ 50 measurements of line-of-sight wind velocity per second
The width of the Doppler spectrum is determined by three elements:
• Instrumental width: this corresponds closely to the ∼ 200 kHz bin width mentioned
above.
• Transit-time broadening: during the conical scan, the beam passes through the aerosol
particles in a timescale of ∼ 10 − 15 µs, independent of the lidar focus setting. The
corresponding broadening is again of order 200 kHz.
• Turbulence broadening: the probing of a significant volume results in a range of Doppler
shifts from parts of the atmosphere that are moving at different speeds (see section
4.3). In general, this contribution increases with turbulence and shear, and occasionally
there is more than one peak in the spectrum as a result. There is potential for using this
broadening to measure and characterise turbulence at a fundamental level.
The turbulence broadeningdominatesexcept underconditionsofvery uniform airflow.High
system sensitivity is of crucial importance for a wind lidar reliant on weak backscatter from
82 DTU Wind Energy-E-Report-0029(EN)